In investigations of lost aircraft and UFO sightings, there is a lot of confusion over what RADAR can and cannot do. The following Frequently Asked Questions will give you an idea of the real capabilities of RADAR:
RADAR (RAdio Detection And Ranging) is a system where a narrow beamed pulse of radio energy is sent out by a transmitter to detect distant objects. When the beam strikes something that can reflect its energy, part of the beam is reflected back to the RADAR set. The receiver picks up this reflected energy and displays it.
The time it took for the reflected energy (moving at the speed of light) to come back indicates the distance of the object, while the direction the RADAR antenna is pointing indicates the direction of the object from the RADAR set. Usually the RADAR antenna rotates slowly to look for objects in all directions.
A blip is a spot of light on a RADAR screen that indicates the RADAR set has detected something. Its position on the screen tells the location of the detected object.
A bogey is an unidentified blip on a military RADAR screen, possibly indicating an enemy aircraft. The word comes from the bogey, or Scottish goblin.
Usually RADAR is a line-of-sight system. The ultra high frequencies used for RADAR usually mean that the RADAR beam behaves like a flashlight beam. It can't go around corners. It also can't bend to follow the curvature of the earth.
The Ballistic Missile Early Warning System (BMEWS) accidentally gave a missile launch warning after it detected the moon (about 250,000 miles away) in 1960, two days after it was turned on. So it can work at least that far. And RADAR has been used to map the unseen surface topography of the planet Venus. But the useful range for ordinary airplane traffic seldom exceeds 100 miles.
The following table shows the minimum altitude (rounded) a RADAR set on the ground (near sea level) can detect a flying object. Below that altitude, the earth's curved surface hides the object from the RADAR:
| Distance from RADAR Set | Minimum Altitude of Object | |||
|---|---|---|---|---|
| Miles | Kilometers | Feet | Miles | Kilometers |
| 50 mi | 90 Km | 1,500 ft | 0.3 mi | 0.5 Km |
| 100 mi | 180 Km | 6,500 ft | 1.2 mi | 2.2 Km |
| 150 mi | 270 Km | 15,000 ft | 3 mi | 5 Km |
| 200 mi | 360 Km | 26,000 ft | 5 mi | 9 Km |
| 250 mi | 450 Km | 40,000 ft | 7.5 mi | 14 Km |
| 300 mi | 550 Km | 80,000 ft | 15 mi | 27 Km |
Note that an object cannot be detected behind intervening terrain (e.g. a mountain).
On the other hand, an object flying directly over the RADAR set is invisible to RADAR because the beam is pointed outward from the station, not straight up.
The above limits on how far ground-based RADAR can track aircraft also limit tracking of aircraft over the ocean to the same distances. Any tracking over open ocean would require a RADAR set on a ship or an aircraft, which would become quite expensive to provide. The planes can still be contacted using radio that can bend over the visible horizon.
Radio waves that can bend over the horizon are long enough to also bend around an airplane without being reflected from it. It can't detect the airplane.
There are RADAR sets that use frequencies that can bend over the horizon and detect objects out to 1000 miles. The US and Australia have such sets. But they don't give precise locations of the objects they detect, just the presence of such objects and their general locations.
That was the intent, but in practice it is not so. RADAR detects things that are not in the sky, and it fails to detect some things that are in the sky. It can also display one object in multiple places, and display an object in the wrong place.
RADAR is intended mainly to detect aircraft, including unmanned aircraft and missiles.
RADAR is also used to detect ships, weather, weather balloons with RADAR targets, satellites, and other miscellaneous items.
A special kind of RADAR (called Doppler RADAR) is used to detect the speeds of objects toward or away from the RADAR set. Examples of these objects include motor vehicles, weather objects, bullets, baseballs, and track-and-field equipment. It can also detect the rotation of a tornado.
Yes. RADAR can detect all of the following, and many more:
Doppler RADAR has its own list of objects it is not supposed to detect. These objects or effects cause the reported speed to be wrong:
This is unusual, but is usually caused by one of the following:
During the early days of RADAR in World War II and the next few years, the causes of unknown targets on RADAR were not known. Someone joked that they might be tracking angels, and the name stuck.
The unknown targets were anomalous targets, which have various causes.
There are several different effects that cause unknown targets to appear on RADARscopes when objects are not actually at those locations:
The RADAR beam bounces off more than one object before returning to the RADAR set. It produces a target that appears farther away than any of the real objects in the multiple reflection.
The RADAR beam bounces off something that is so far away that the set has sent out more pulses before the echo comes back. It is displayed as though it belongs to a later pulse, much closer than the real object is.
Leakage from the RADAR antenna near the beam strikes an efficient corner reflector (a place where three metal surfaces all meet at right angles, making an inside corner). The corner reflector then reflects this leakage back to the RADAR antenna, producing a signal strong enough to make a blip.
Objects that make good corner reflectors include rectangular dumpsters, dump trucks, open-back box trucks, and railroad hopper cars.
The corner reflector can produce a blip in the wrong compass direction, and it can participate in a multiple reflection (above) or a side lobe blip (below).
Some of the RADAR beam leaks out of the antenna in the wrong direction due to the shape of the antenna. A very reflective target can produce a blip when the energy of the side lobe of the antenna hits it. That object makes multiple blips, all the same distance from the set.
Objects tall enough to be struck by the RADAR beam will appear as blips on the screen. These include tall buildings and antenna towers. But they are easy to identify on the screen, because they never move.
The RADAR beam can get close enough to the water within the closest 50 miles to pick up ships. The interface between a broadside ship and the water also makes a good corner reflector.
Within the first few miles of the RADAR set, many objects on the ground are struck by the beam and produce blips, causing a continuous large target called ground clutter. Objects cannot be tracked through ground clutter.
A small object that is present at the location shown by the RADAR can produce the RADAR blip without being seen visually by ground or air observers. Such objects include metal-tagged birds, escaped aluminized Mylar toy balloons, and prank UFO balloons.
In addition to producing areas of weather clutter, a concentrated core in a storm can produce a blip similar to one made by an aircraft.
In wartime, RADAR jammers are transmitters that usually cover the screen with lines, dots, or a continuous smear that makes the RADAR set useless for detecting the enemy.
In wartime, chaff (also known as window) is metal foil dropped from aircraft or missiles that covers the RADAR screen with blips. The RADAR operator can't tell which blips are real aircraft and which are chaff.
Electrical devices can produce unwanted radiation at RADAR frequencies, especially if something goes wrong. Electrical shorts in power lines give off broadband blasts of energy, causing random blips that do not repeat. Other devices inadvertently producing RADAR frequencies usually do not make sharp blips. They make smeary patches on the screen.
Anomalous propagation can cause interference between two RADAR sets that are normally far enough apart that they don't affect each other. Such blips usually jump around the screen because the sets are not synchronized with each other. Operators have called these targets "running rabbits."
Anomalous propagation is an atmospheric condition that bends RADAR beams. This causes objects that would not normally appear on RADAR to show up on the screen. Often the RADAR beam bends downward, striking objects that are on the ground, are on the ocean, or are normally beyond the RADAR horizon.
If the RADAR receiver gain (or the AGC gain) is inadvertently turned up too high, many marginal RADAR targets can appear as planelike blips. These include birds, swarms of insects, weather targets, and other objects that normally do not display as blips.
Anomalous propagation is an atmospheric condition where the RADAR beams, instead of moving in straight lines, follow curved or bent paths. This causes objects that the beam would not normally strike to show up on the screen. It works similarly to a visual mirage, and is caused by the same atmospheric conditions.
Anomalous propagation occurs when different layers of air with different temperatures and humidities come together to distort the paths of light rays and RADAR beams. Often a temperature inversion causes anomalous propagation. When stars near the horizon appear to dance around and change colors, it is a sign that anomalous propagation is occurring.
Anomalous propagation can either reflect or refract the RADAR beam, causing it to go in an unexpected direction. Usually it is bent down, where it strikes objects on the ground and aircraft beyond the RADAR horizon. Sometimes it focuses the beam, enabling it to pick up objects far beyond the normal range of the RADAR set.
A temperature inversion is a layer of reversal of the normal decrease in temperature with an increase in altitude. It can cause light rays and RADAR beams to bend.
There are several kinds of RADAR intended for several different purposes:
MTI stands for Moving Target Indicator. This is a system devised in the 1950s to remove all stationary targets from the RADAR screen. It originally worked using a mercury delay line to delay each returned RADAR reflection the exact time between transmitted pulses. If a returned reflection appears at exactly the same time as the previous returned reflection, the blip so produced is removed.
Newer RADAR sets use computers to accomplish the same result. That result is to remove any target that is not moving from the RADAR screen.
There are several different effects that can cause this:
Primary RADAR is RADAR that displays every echo returned to it as a blip. It detects aircraft by detecting a pulse from the RADAR set that is reflected from the metal parts of the aircraft. The aircraft needs no special equipment to be detected.
Skin-paint RADAR and primary RADAR are names for the same thing.
Secondary RADAR is a part of the RADAR system for commercial and military aircraft that identifies the aircraft to any RADAR set so equipped, and also reports the aircraft's altitude. A transponder device on the aircraft receives the RADAR pulse and returns the plane's identity and altitude.
A transponder is an electronic device that receives the RADAR pulse and sends back a coded pulse giving information about the airplane or the flight. The RADAR set displays info about the plane next to the blip.
This was originally developed by the military as "IFF" (Identification Friend or Foe) to show which planes are friendly. After the technology was perfected, the concept was transferred to commercial aircraft. But very few private aircraft have transponders.
Transponder RADAR is a RADAR system that depends on transponders to display blips. Objects without transponders do not show up on this kind of RADAR. The advantage is that the RADAR set transmitter can operate with a much lower power, preventing the unwanted effects on electric equipment near the RADAR that primary RADAR causes.
This is a combination of primary (skin-paint) RADAR and transponder RADAR into one RADAR set. It is usually switch-selectable to use the primary RADAR, the transponder RADAR, or both.
The advantages are:
The disadvantages are:
They made little paper pointers with flight numbers and put them on the RADAR screen to identify the blips. As the planes moved, they moved the paper pointers.
They also had paper strips they got from flight schedules. Sometimes these were made with the paper pointers printed on one end.
That depends on what kind of RADAR he is using:
That depends on what kind of RADAR he has selected:
A height finder is a special RADAR with an antenna that rapidly tilts up and down. The circuitry in this RADAR determines the heights of the targets by comparing the strengths of the echoes at different antenna tilt angles. Various methods were used to display the height.
On the exclusively transponder screen, there aren't any. Every item on the screen is identified by a transponder code from a known aircraft.
On the primary RADAR screen, ALL of the blips are "unidentified." The air traffic controller has to keep track of blips with little paper pointers. But he keeps track of only the aircraft he has under his control. The rest are aircraft under control of a different controller, aircraft flying under visual flight rules, and an occasional anomalous target.
On the primary/transponder RADAR, the commercial and military planes appear on the screen with identifying tags and altitude information. The remaining blips are private planes without transponders, with some occasional anomalous targets thrown in.
No. It is not.
The job of an air traffic controller is to keep those blips from crashing into each other. He does that by guiding the aircraft he is contacting and controlling away from the other blips.
He also knows that most private aircraft do not have pressurized cabins or oxygen, so they stay below 15,000 feet. The commercial flights he controls have pressurized cabins, so they stay above 18000 feet. So he can ignore most of the unidentified blips.
That depends on the true nature of the UFO.
A UFO is an Unidentified Flying Object, not a vehicle from outer space.
A UFO on RADAR can be an unexpected aircraft, or any other kind of target. RADAR operators call these targets Uncorrelated Targets (UCT):
There are several ways:
A misadjustment of the automatic gain control (AGC) on the RADAR receiver can cause weak anomalous targets to look like aircraft blips (the UFOs) when no real aircraft is present. This often occurs at night, when traffic is sparse. When a real aircraft enters the range of the RADAR, its strong target turns the gain down where it belongs, and the anomalous targets disappear.
On one early RADAR set, an upgrade moved the gain control to a different control knob, and changed the old gain knob to become the automatic gain control adjustment. But operators often forgot. By force of habit, the operators turned the old knob. Misadjusting this knob caused the weak anomalous targets to turn into planelike blips when no real aircraft was present. It also makes any MTI device stop blocking stationary targets.
The set must be recalibrated to remove this effect.
It depends on the type of RADAR being used:
A combination of methods is used:
There are several reasons:
Such transmissions prevent RADAR from correctly identifying other aircraft on the screen.
Even if the transponder is undamaged and still has power, the RADAR beam intentionally avoids scanning anything on land.
Even if the transponder is undamaged and still has power, radio and RADAR waves don't work under water.
Even if a beacon is undamaged, radio and RADAR waves don't work under water.
It's not a matter of the design of the device. It is the nature of water. Because water (especially salt water) is a conductor of electricity, radio and RADAR waves cannot pass through it. It is the same as being inside a steel building that radio waves can't penetrate. Without an antenna above the surface of the water (or outside the steel building), radio communication can't occur.
Several methods are used:
The aluminum foil was traveling with the car, making it a better target. Chaff leaves the aircraft, producing multiple targets. Also, police RADAR looks for speed, not a specific target.
Note that police also now have LIDAR, which uses LASER light instead of RADAR.
Did he cover everything? The RADAR can bounce off the undercarriage, the headlights, the tail lights, the wheels, and the car interior. He could not RADAR-proof it and still have a drivable street-legal vehicle.
Police also now have LIDAR, using LASER light instead of RADAR.
This was probably a multiple reflection of the RADAR beam from multiple objects. This often happens when a corner reflector is on a vehicle going the other way (e.g. an open-back box truck).
There are several possibilities:
In one case, people were wrongly ticketed in only one place, and only on friday nights. There was a drag racing track next to the road, but on the other side of a tall substantial wood fence. They held racing every friday night. But the entrance to the track was on another road. The police thought the loud roars came from the cars they stopped (by the time the car was stopped, the race was over and the track was being made ready for the next heat). The RADAR reading seemed to confirm it. Finally someone who knew told the policeman who stopped him, "There's a race track on the other side of that fence."
There are several ways a false reading can show up on a police RADAR:
They were not looking for UFOs or Russian planes. The RADAR men were doing research to identify what kinds of objects or events caused RADAR returns where no aircraft were known to be. So they sent airplanes to the places where the targets were displayed, to put eyes on the area and find out what was there.
Unfortunately, the pilots were not told that this was merely RADAR research (it was classified). They just answered all of the questions they were asked by the RADAR operators on what they saw while they were in the target area. But they didn't know that they were doing research, instead of searching for potential invaders. Many UFO stories came from these flights.
This was one of the cases described in the question above, asked as:
The RADAR sets had just been retrofitted to have AGC and MTI. The upgrade moved the manual gain control, making the old control the AGC control. The RADAR operator turned the old knob by mistake, uncalibrating the RADAR set. The set then displayed weak targets as UFOs.
In addition, anomalous propagation was occurring on those nights. One pilot found himself aimed at a steamboat in the Potomac each time he tried to intercept a UFO blip over the White House.
Note that most of the visual sightings were of Mars, which was bright red in the sky at the time.
This was another of the cases described in the question above, asked as:
The RADAR sets had again just been retrofitted to have AGC and MTI. The upgrade moved the manual gain control, making the old control the AGC control. The RADAR operators again turned the old knob by mistake, uncalibrating the RADAR set. The set then displayed weak targets as UFOs.
In addition, anomalous propagation was occurring on those nights. Some of the objects were meteors from the Perseid meteor shower, detected visually or as multiple-trip echoes on RADAR.
Most of the visual sightings were airplanes. Some of them were probably the T-33s or the Venom fighters sent up to identify the UFOs. Some were meteors.
This was an extreme case of visual and RADAR anomalous propagation, which turned ordinary stars, planets, and aircraft lights into strange-looking bright lights, and multiplied weak RADAR weather targets into large planelike blips.
Essentially, the effects of unusual weather caused this rash of sightings. But there was also an unscheduled flight of 4 Navy aircraft the Air Force did not know about.
This was a then-classified series of stealth technology tests. Various stealth devices were tested for their ability to hide aircraft or produce blips where aircraft were not present.
The problem was that none of the officials at any of these air bases were told the truth, either at the time or afterwards. They thought they had unexplained UFO sightings, instead of these secret tests. The truth was revealed 5 years later.